This is the first application filed for the present invention.
The invention relates to power conversion and, in particular, to methods and apparatus for delivering electrical power to low voltage electrical loads with an enhanced power conversion efficiency.
When using power converters for powering electrical loads, such as low-voltage incandescent lamps, excessive heating of power converter components is common due to inefficiencies in power conversion. An exemplary prior art power converter for a low-voltage incandescent lamp is taught in U.S. Pat. No. 5,036,253 that issued on Jul. 30, 1991 to Nilssen. The power converter is adapted to be driven from an electrical utility outlet that provides 60 Hz, alternating current (AC). The power converter provides at an output, a relatively high-frequency (30 kHz) substantially squarewave signal. The output signal is generated by an oscillating power driver stage of the power converter that is driven by a rectified AC input voltage derived via full-wave rectification of the 60 Hz utility power voltage. The oscillating power driver stage is of a type that has to be triggered into oscillation. However, once triggered, it will continue to oscillate, but only for as long as the instantaneous voltage magnitude of the rectified AC input exceeds minimum oscillation voltage level. The rectified AC input voltage falls to zero magnitude at a rate of 120 Hz. The oscillating power driver stage stops oscillating with each such zero level dip, as the rectified AC input voltage drops below the minimum oscillation voltage level. The oscillating power driver stage has to be re-triggered after each rectified AC voltage dip. Triggering is accomplished by a diac. The oscillating power driver stage is triggered into oscillation only after the rectified AC input voltage has risen above a turn on voltage of the diac (about 32V).
The output modulated envelope of conventional power converters is characterized by a discontinuous sinusoidal wave envelope. This discontinuity is reflected back into the utility power lines as a current drawn from the utility power lines, distorting the sinusoidal wave variation thereof. The result is that the power conversion factor and thus the efficiency of the power converter is reduced.
FIG. 1 illustrates a prior art power converter 100 supplied with input power, such as AC electrical utility power 102. An optional input stage 104 filters the utility power to suppress transients. The optional input stage 104 may also be adapted to suppress high frequency components reflected back from the power converter 100. A rectification stage 106 is used to convert the AC input power to rectified AC input power. The oscillating power driver stage 108 is supplied with the rectified AC input power to generate a high frequency substantially squarewave output. The oscillation of the oscillating power driver stage 108 is sustained by an oscillation feedback signal 110 derived from the high frequency squarewave output using a feedback signal pickup stage 112. The high frequency squarewave output is further conditioned by an output stage 114 before being applied to an electrical load 116.
If an electrical fault occurs, conventional protection circuits of re-triggerable power converters prevent the diac from triggering the oscillation. Once the oscillating power driver stage 108 is oscillating and an electrical fault is detected, the oscillating power stage 108 supplies power into the electrical fault until the rectified AC input voltage drops below the minimum oscillation voltage level required to stop the oscillation. Consequently, for a period of time conventional power converters will operate into an electrical fault condition, thus stressing the components of those power converters.
Conventional fault protection circuits are constructed to delay for a period of time before attempting to restart oscillation after a fault occurs. If the electrical fault condition persists, the oscillating power driver stage 108 is triggered into oscillation and operates into the electrical fault condition until the rectified AC input voltage again drops below the minimum oscillation voltage level, inducing further desirable stress on power converter components, and potentially leading to a build-up of heat, power loss and poor power conversion efficiency.
There therefore exists a need to enhance power conversion efficiency of low-voltage power converters, and provide a more effective fault protection mechanism to ensure that power converter components are not unduly stressed by a fault condition.
It is therefore an object of the invention to provide a power converter that delivers stored energy to an oscillating power driver stage during periods of low input voltage from a rectified alternating current input.
It is another object of the invention to suppress power delivery to an electrical load by the power converter in the event of a detected electrical fault.
It is a further object of the invention to suppress oscillation of an oscillating power driver stage of the power converter on detection of an electrical fault.
It is an further object of the invention to suppress an oscillation feedback signal driving an oscillating power driver stage of the power converter on detection of an electrical fault.
In accordance with the invention, there is therefore provided a method of supplying electrical power to an oscillating power driver stage of a power converter. The method comprises a first step of accumulating electrical energy in an electrical energy store during periods of high input voltage from a rectified alternating current (AC) input. In a second step, power is supplied from the energy store to the oscillating power driver stage during periods of low input voltage from the rectified AC input, so that power is continuously supplied to the oscillating power driver stage to enable uninterrupted oscillation of the power converter while the AC input voltage is supplied to the power converter.
The method further comprises a step of accumulating electrical energy in the energy store to an extent required to provide power to the oscillating power driver stage during a duration of low input voltage from the rectified AC input. The extent to which the energy is accumulated ensures minimal power drain to enhance a power conversion factor of the power converter. Alternatively, the method further comprises a step of charging the energy store at a controlled rate to charge the energy store to an extent required to provide power to the oscillating power driver stage during the periods of low input voltage from the rectified AC input. The controlled rate at which the energy store is charged ensures minimal power drain to enhance a power conversion factor of the power converter.
In accordance with a second aspect of the invention, there is provided a power converter for supplying electrical power to an electrical load. The power converter comprises an electrical energy store charged during periods of high input voltage from a rectified AC input to supply at least a portion of the stored charge to an oscillating power driver stage of the power converter during periods of low input voltage from the rectified AC input. The energy store preferably comprises a capacitor for storing electrical energy required to sustain oscillation of the oscillating power driver stage during periods of low input voltage of the rectified AC input. The capacitor may have an energy storage capacity in excess of the electrical energy required to sustain oscillation of the oscillating power driver stage during periods of low input voltage of the rectified AC input. The power converter may also include a Zener diode coupled across terminals of the capacitor to control an upper limit on the amount of electrical energy stored by the capacitor to sustain oscillation of the oscillating power driver stage during periods of low input voltage from the rectified AC input, and to limit power drain to enhance the power conversion factor of the power converter. The power converter may further include a charging rate limiter to charge the energy store during periods of high input voltage from the rectified AC input at a charging rate selected to store an amount of electrical energy required to sustain oscillation of the oscillating power driver stage during periods of low input voltage of the rectified AC input. The charging rate limiter may comprise a resistor for limiting the current flow, for example. The power converter may also further comprise a diode coupled across the charging rate limiter to couple at least a proportion of the energy stored directly to the oscillating power driver stage during periods in which the voltage of the rectified AC input falls below that of the energy store to sustain uninterrupted oscillation of the oscillating power driver stage during periods of low input voltage from the rectified AC input.
In accordance with a third aspect of the invention, there is provided a method for suppressing the operation of a power converter supplying power to an electrical load to minimize power dissipation during detected electrical faults. The method comprises a first step of generating a fault signal on detecting the fault. A latch is activated in response to the generated fault signal. Activation of the latch suppresses an oscillation feedback signal that promotes the oscillating power driver stage into oscillation. The method further comprises a step of diverting current flow used to derive the oscillation feedback signal from an output of the oscillating power driver stage, the diversion being effected via an auxiliary transformer coil associated with the drive transformer.
In accordance with a fourth aspect of the invention, there is provided a fault protected power converter for suppressing oscillation of an oscillating power driver stage on detecting an electrical fault. The power converter comprises a fault detector providing a fault signal, a latch having a default inactive state and a promoted active state, the promoted active state providing a shutdown signal, and an auxiliary winding associated with a drive transformer providing an oscillation feedback signal to the oscillator power driver circuit. The auxiliary coil, when driven by the shutdown signal, diverts current flow used to derive the oscillation feedback signal from an output of the oscillating power driver stage during detected electrical faults.
The oscillating power driver stage may include the fault detection circuit. The oscillating power driver stage may further comprise a fault signal generation circuit.
Advantages of the invention include power savings, an extended service life for power converter components, reduced heat output in general, and reduced heat and surge stress on power converter components during electrical faults.